Features

Galactic Disk

For most galaxies, and particularly spiral galaxies like the Milky Way, the galactic disk is a 50,000-lyr radius, 1000-lyr thick region containing the majority of the baryonic mass of the galaxy. Therough the middle of this region is the galactic plane.

The stellar density, the number of stars contained in any 1-lyr cube, varies over this plane, being highest near the galactic core and the middle of the spiral arms, and the lowest, between the spiral arms and toward the galactic rim. Stellar density is measured with units of stars per cubic lyr, which is presented as: stars/cu-lyr. Although the stellar density within a spiral arm is, on average, high, it is not uniform, and there is still significant variation. Sol is located in the Rift of Sol, an area of low stellar density, with just 0.003 stars/cu-lyr. 500 lyrs rimward of Sol is the middle of the Bellatrix Clouds (part of the Taurean Rim), where stellar densities are in the order of 0.08 stars/cu-lyr.

Open Clusters

Open clusters are a collection of stars numbering from 100 or so up to 50,000, which are mutually gravitationally bound. They are recogniseable as an area of high stellar density with respect to the background stellar density of the galactic disk.

Open clusters become more dispersed and diffuse with age, but retain a high-density core (between 3 and 4 lyrs accross) and a relatively lower density corona (between 20-150 lyrs radius). Within the core stellar densities are typically around 1.5 stars/cu-lyr. This is a similar density to that found in the galactic core. In the corona the density begins at around 0.2 stars/cu-lyr, but is gradually eroded after about 2 billion years to 0.08 stars/cu-lyr, which is about the stellar density of a high-density region in a galactic arm.